Many continuously variable drive units (CVU) utilize a pair of spaced adjustable sheaves or pulleys that are drivingly connected by a flexible torque transmitter such as a belt. In most modern CVUs, the belt is metal member comprised of a plurality of metal plates maintained in a continuous loop by a plurality of thin metal bands. These metal belts permit a large amount of torque transfer as compared with conventional rubber V-belts.
The diameters about which the belt is trained on the sheaves determines the transmission ratio between the input and output shafts connected therewith. For example, if the diameters are equal, a one to one ratio is present, if the input sheave diameter is smaller than the output sheave diameter, an underdrive ratio is provided and if the input sheave diameter is greater than the output sheave diameter, an overdrive ratio is presented. During operation, the sheave diameters are controlled to provide a continuously variable ratio as opposed to a step ratio common to most power transmissions. The ratio between the input and output shafts is varied from an underdrive at vehicle launch to an overdrive ratio at cruising speeds.
The ratio between the shafts may be held at any of the available ratios during vehicle operation. The ratio between the sheaves is generally maintained by hydraulic pressure applied to a moveable sheave in each of the pulleys. The pressure is supplied by a control system which maintains a high pressure on one of the pulleys, usually the output pulley, and varies the pressure on the other pulley. At least one prior art control incorporates a position feedback indicator or valve to control the pressure in the input pulley. An electronic control issues a ratio command and the pressure at the input pulley is varied relative to the output pulley until the commanded ratio is present. The feedback indicator informs the control when the commanded ratio is achieved.
Other prior art control use direct pressure control to establish the ratio between the input and output pulleys. The output pulley is pressurized at a level determined by the torque load and the input pulley control pressure is varied in accordance with a look-up table which stores the pressure values for each ratio. The control issues a command to adjust the pressure to provide the calculated pressure for the requested ratio.
While both of these systems will provide a ratio control in a CVU, they both have some disadvantages as to cost and complexity. The position feedback controls require mechanical elements disposed in abutment with the input pulley. These systems also require a mechanical connection between the feedback member and a valve member disposed to control the pressure on the pulley. The direct pressure systems use an electronic processing unit and need sufficient memory space for the tables used by a digital computer for controlling the CVU and software to provide the lookup function. Also the currently available direct pressure control systems do not make non electrical accommodations for fluctuating pressure in the output pulley control pressure which can occur during vehicle operation.